In rats, morphine (s.c.) and beta endorphin (i.v.t.) administration decreases the turnover rate of gamma-aminobutyric acid (GABA) in the N. caudatus, but increases the turnover rate of GABA in globus pallidus and substantia nigra. The effects of morphine on the turnover rate of GABA are reversed by naltrexone. There is no dopaminergic involvement in the turnover rate of GABA. The increases of GABA turnover in globus pallidus seems to play an important role in the morphine induced catalepsy. In fact, musimol, a GABA receptor stimulating agent, when injected bilaterally into the globus pallidus mimics morphine catalepsy. Thus, the action of morphine and beta endorphin on the turnover rate of GABA in the caudate and globus pallidus may be due to an effect on opiate receptor located in the head of the caudate nucleus. Since the GABA turnover in caudate decreases after opiate receptor administration and after surgical procedures which destroy the cortico-striatal afferents, it is possible that opioid neurons modulate the afferents to caudate and through these mechanisms modify the turnover rate of GABA.